WASHINGTON - Working against a midnight deadline, negotiators for the White House and congressional Republicans narrowed their differences Monday on legislation to avert across-the-board tax increases.

Congressional officials familiar with talks between Vice-President Joe Biden and Senate Republican leader Mitch McConnell said one major remaining sticking point was whether to postpone spending cuts that are scheduled to begin on Jan 1.

Republicans want to replace across-the-board reductions with targeted cuts elsewhere in the budget, while the White House and Democrats want to offset at least some of the so-called sequester with the revenue from tax increases. Senate Democrats were pushing hard against a GOP proposal for just a three-month delay in the across-the-board cuts.

At the same time, Democrats said the two sides were closing in on an agreement over taxes. They said the White House had proposed blocking an increase for most Americans, while letting rates rise for individuals with incomes of $400,000 a year and $450,000 for couples, a concession from President Barack Obama's campaign call to set the levels at $200,000 and $250,000.

Any overall deal was also likely to include a provision to prevent a spike in milk prices with the new year, extend unemployment benefits due to expire and protect doctors who treat Medicare patients from a 27 per cent cut in fees.

Despite the movement, Senate Majority Leader Harry Reid warned that differences remained without spelling out what they were and said co-operation would be needed by both sides.

"Negotiations are continuing as I speak," said Reid, D-Nev., as the Senate began an unusual New Year's Eve session. "But we really are running out of time. Americans are still threatened with a tax hike in just a few hours."

Senator Tom Harkin, D-Iowa, on the liberal end of the U.S. political spectrum, took to the Senate floor after Reid to warn Democratic bargainers against lowering levies on large inherited estates and raising the income threshold at which higher tax rates would kick in.

"No deal is better than a bad deal. And this look like a very bad deal the way this is shaping up," said Harkin.

He suggested instead letting tax rates revert to the higher levels that existed when the economy was strong under President Bill Clinton, adding, "I ask, what's so bad about that?"

Both the House and Senate were meeting on the final day of the year, although there was no expectation that legislation could be approved by both houses by midnight, even if a compromise deal is reached.

Instead, the hope of the White House and lawmakers was to seal an agreement, enact it and send it to Obama for his signature before taxpayers felt the impact of higher income taxes or federal agencies began issuing furloughs or taking other steps required by spending cuts.

Regardless of the fate of the negotiations, it appeared all workers would experience a cut in their take-home pay with the expiration of a two-year cut in payroll taxes.

Officials who described the negotiations did so on condition of anonymity, citing the confidential nature of the discussions.

McConnell's spokesman, Don Stewart, said the Kentucky lawmaker and Biden "continued their discussion late into the evening and will continue to work toward a solution." Underscoring the flurry of activity, another GOP aide said the two men had conversations at 12:45 a.m. and 6:30 a.m. Monday.

Unless an agreement is reached and approved by Congress by the start of New Year's Day, more than $500 billion in 2013 tax increases will begin to take effect and $109 billion will be carved from defence and domestic programs

Though the tax hikes and budget cuts would be felt gradually, economists warn that if allowed to fully take hold, their combined impact — the so-called fiscal cliff — would rekindle a recession.

"This whole thing is a national embarrassment," Sen. Bob Corker, R-Tenn., said Monday on MSNBC, adding that any solution Congress would swallow at this late stage would be inconsequential. "We still haven't moved any closer to solving our nation's problems."

In a move that was sure to irritate Republicans, Reid was planning — absent a deal — to force a Senate vote Monday on Obama's campaign-season proposal to continue expiring tax cuts for all but those with income exceeding $200,000 for individuals and $250,000 for couples.

In one sign of movement on Sunday, Republicans dropped a demand to slow the growth of Social Security and other benefits by changing how those payments are increased each year to allow for inflation.

Obama had offered to include that change, despite opposition by many Democrats, as part of earlier, failed bargaining with House Speaker John Boehner, R-Ohio, over a larger deficit reduction agreement. But Democrats said they would never include the new inflation formula in the smaller deal now being sought to forestall wide-ranging tax boosts and budget cuts, and Republicans relented.

"It's just acknowledging the reality," Sen. Susan Collins, R-Maine, said of the GOP decision to drop the idea.

There was still no final agreement on the income level above which decade-old income tax cuts would be allowed to expire. While Obama has long insisted on letting the top 35 per cent tax rate rise to 39.6 per cent on earnings over $250,000, he'd agreed to boost that level to $400,000 in his talks with Boehner. GOP senators said they wanted the figure hoisted to at least that level.

Senators said disagreements remained over taxing large inherited estates. Republicans want the tax left at its current 35 per cent, with the first $5.1 million excluded, while Democrats want the rate increased to 45 per cent with a smaller exclusion.

The two sides were also apart on how to keep the alternative minimum tax from raising the tax bills of nearly 30 million middle-income families and how to extend tax breaks for research by business and other activities.

Republicans were insisting that budget cuts be found to pay for some of the spending proposals Democrats were pushing.

These included proposals to erase scheduled defence and domestic cuts exceeding $200 billion over the next two years and to extend unemployment benefits. Republicans complained that in effect, Democrats would pay for that spending with the tax boosts on the wealthy.